Most select agents such as orthopoxviruses first come into contact with hosts via mucosal epithelial surfaces. The first line of defense to these infectious agents are cells of the innate immune system, including epithelial cells, M cells, dendritic cells, polymorphonuclear cells, macrophages, natural killer (NK) cells and TCRgamma-delta cells. Understanding how select agents trigger innate immune cells to recruit antigen-specific adaptive immune responses is a high priority for developing novel therapies and vaccine adjuvants for select agent infections. CD7 is a 40kD Ig superfamily molecule on TCRalpha-beta and TCRgamma-delta T cells and NK cells. Ligation of CD7 on cells of the innate immune system directly induces TCRgamma-delta and NK cell calcium flux, proliferation, and increases NK killing. These effects are specific for NK and TCRgamma-delta T cells compared to TCRalpha-beta T cells. The CD7 ligand (CD7L) is a recently discovered secreted 20kD Ig family molecule termed K12 (SECTM1) that is expressed by breast, lung, gut and thymic epithelial cells as well as by polymorphonuclear cells and other non-lymphoid cells. IFN-gamma is a potent inducer of K12 secretion in epithelial cells. Whereas CD28 (T cells) and B7.1/B7.2 (APCs)is a potent "co-stimulatory" receptor-ligand pair for TCRalpha-beta T cells, a similar "immune response enhancing" receptor-ligand pair that augments NK and TCRgamma-delta T cell activation is not known. Recent work from our laboratory on CD7 and its CD7L (K12) suggests that CD7 ligation on NK and TCRgamma-delta cells may be an important receptor-ligand pair for activation of the innate immune system, and serve as a bridge between triggering of the innate and acquired/adaptive immune systems. We have found that IFN-gamma stimulates K12 production by lung and gut epithelial cells, and that epithelial cell K12 production can be modulated by vaccinia virus. In this project we will determine the mechanisms of cytokine and vaccinia virus regulation of epithelial cell production of CD7L (K12) mRNA and protein production by respiratory cell lines and primary epithelial tissue explants in vitro. We will determine the ability of CD7L (K12) to serve as a therapeutic immune stimulant for the innate immune system. Lastly, we will determine the ability of recombinant mouse K12-Ig fusion protein to serve as a vaccine adjuvant in a Balb/c mouse cowpox challenge model with a prototype orthopoxvirus subunit vaccine formulation.